Electrodeposition of zinc



Patented Sept. 10, 1940 ELECTBODEPOSITION F ZINC Richard 0. Hull, Lakewood, Ohio, assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application April 27, 1937,

Serial No. 139,203

6 Claims. This invention relates to the electrodeposition of zinc, and is particularly directed to the use of zinc anodes which contain small amounts of calcium.

In processes for the electrodeposition of zinc it is customary to maintain the zinc content of the zinc plating baths by the use of zinc anodes, but considerable difliculty is experienced by reason of the fact that the anode efficiency is greater than the cathode efiiciency. In other words, the zinc metal of the anode is dissolved in the bath more rapidly than zinc metal is deposited from the bath onto objects being plated. As a result of the rapid solution of the anode the bath soon contains excessive amounts of zinc compounds.

For the best results it is desirable that a zinc plating bath remain as near as possible to a definite formula, and changes in bath composition resulting from the rapid solution of zinc anodes cause considerable difilculty. In the recently developed processes for the electrodeposition of bright zinc deposits it is particularly important that the bath composition be held rigidly within limits if the best possible resultsare consistently to be obtained.

In addition to the dimculties arising from the rapid solution of zinc anodes in zinc plating baths, further diiilculties result from the fact that the anodes do not dissolve evenly and from the fact that a black sludge is formed which floats in the bath and on the surface of the bath. The presence of black sludge is particularly disadvantageous in processes for the deposition of bright zinc from cyanide baths since this black sludge interferes with the production of a smooth and bright plate.

When a zinc plating bath has such an amount of zinc dissolved therein that it becomes necessary to correct the bath composition, this can be done by'removing some of the zinc. Or, as is more customarily the practice, the excess of zinc compounds may be diminished by discarding a portion of the solution and correcting the com-v position of the remaining solution by the addition of bath constituents other than zinc compounds. It will of course be apparent that such procedures are uneconomical and time-consuming.

It has' been proposed to compensate for the high anode efliciency of zinc anodes in cyanide baths by replacing some of them with insoluble iron or steel anodes. This expedient results in a diminution of the amount of zinc dissolved per unit of electrical current passed into the bath, and in effect reduces the anode efiiciency.

Unfortunately this type of procedure is not entirely satisfactory because when the bath is standing idle the iron or steel anodes accelerate the rate of corrosion of the zinc anodes, thus causing the zinc to dissolve rapidly and unevenly during periods when the bath is not in operation. It is, moreover, generally accepted that insoluble anodes cause destructive oxidation of cyanide during electrolysis. Such oxidation is considered to destroy the valuable cyanides and to produce deleterious carbonates.

It has also been proposed to use zinc-aluminum alloy anodes. By such use of aluminum in a zinc anode the anode efiiciency is slightly decreased since zinc-aluminum alloys are slightly ly less soluble than pure zinc metal. This proposed use of zinc-aluminum alloy anodes was primarily suggested for acid-zinc plating baths 2 and has been found most useful in this relation. Unfortunately the use of aluminum in zinc anodes does not solve the problem of maintaining zinc plating baths at a constant composition.

The effect of adding alumimun is too small to secure a balance of anode and cathode eillciencies. This is particularly true when the anodes are used in cyanide baths, perhaps because the aluminum is alkali soluble. I

I have found that the difliculties heretofore experienced in the art may largely be overcome by the addition of moderately small amounts of calcium to zinc anodes. By selecting a proper amount of calcium, anodes having an efiiciency corresponding closely to the cathode efliciency of the baths in which they are to be used may readily be produced; By the use of such anodes it .is possible to maintain the balance of bath constituents for amuch longer period than has heretofore been pracicable, particularly with produce a minimum amount of black sludge."

Particularly whenrelatively large amounts of calcium, say in the range around 0.5%, is used,

the amount of black sludge is" markedly de- 65,

creased. In any event the presence of calcium in the anode causes any black sludge which does 'form to settle directly to the bottom of the bath and the disadvantage heretofore encountered of the black sludge floating in the bath and on the surface of the bath is obviated. The action of calcium in promoting even corrosion and in preventing deleterious black sludge formations makes the zinc-calcium alloy anodes of my invention particularly valuable for use in cyanide-zinc plating baths for the production of bright smooth deposits.

The action of calcium in the zinc-calcium alloys'of my invention is entirely different from the action of metals such as aluminum. Aluminum, for instance, is itself soluble in both acid and cyanide-zinc plating baths and zinc alloys containing it are likewise soluble, tho they dissolve slightly less rapidly than pure zinc.

Calcium on the contrary is not soluble in cyanide-zinc plating baths. The calcium in the zinc-calcium anodes of my invention apparently causes the formation of a film of alkaliinsoluble calcium compound on the surface of anodes which are in use. This film polarizes the anodes,in a measure resisting the passage of current between the anode'and the bath. The effect of the calcium is directly proportional to the amount present, and by the use of a suitably large amount of calcium, zinc-calcium alloys of almost any desired efliciency can be made.

Electrolytic zinc anodes for use in cyanidezinc plating baths normally have an anode efficiency of 100 to 105%, while the normal cathode efficiency in similar baths is around 80 to The anode efficiency of such electrolytic zinc anodes may be reduced to fall within the range of cathode efilciencies by the addition of calcium to the zinc metal, for instance, 1% of calcium being suificient under one setof conditions to give an electrolytic zinc-calcium alloy anode having an anode efliciency of 85%. It is of course particularly desirable to be able to reduce the anode efliciency of electrolytic zinc anodesv because the high purity of this type of anode renders it desirable for bright zinc processes.

Many zinc anodes howeverare not made from electrolytic zinc but are made of somewhat less pure zinc obtained by distillation methods. These anodes are much used for zinc plating, and they have found considerable application in the plating of bright zinc from cyanide-zinc plating baths. These less pure anodes. dissolve somewhat more slowly than do the electrolytic anodes, the usual anode efiiciency being around for commercial grade zinc anodes.

Considerably smaller amounts of calcium are suificient to reduce the anode efliciency of zinc anodes to the required amount when commercial zinc from distillation processes is used. For instance 0.6% of calcium was suiiicient to give a zinc-calcium anode having an efiiciency of 85% when the zinc was of a commercial grade obtained by distillation. f

The use of calcium is particularly advantageous with the use of anodes which are less pure than the electrolytic zinc because the calcium tends to reduce the amount of black sludge formed and further causes this sludge to drop to the bottom of the tank where it cannot interfere with the bright plates. As a result, it is entirely feasible to produce brilliant zinc plates from cyanide-zinc plating baths using commercial grades of zinc obtained by distillation if calcium is incorporated in the metal according to the teachings of my invention.

It will readily be understood that various other metals may be included in the zinc anodes in conjunction with calicum in accordance withthe practices already known in the art. I may for instance use calcium jointly with aluminum or mercury, or ,I may use all three to obtain their joint effects.

It will further be apparent that since the impurities present in commercial grades of zinc cooperate with calcium to produce anodes of low efliciency it may conceivably be advantageous in some circumstances to add such impurities jointly with calcium to zinc metal which, by reason of its higher purity, dissolves too rapidly and would require an excessive amount of calcium to obtain the desired anode efliciency. This is particularly true because of the present high price of calcium metal.

In order that my invention may be better understood, the following examples are given:

Example I Molten zinc obtained in a zinc distillation process was transferred in the molten state to a gasfired graphite crucible and the temperature raised from about 450 0. up to a temperature of about 600 C. Five-tenths per cent of calcium was then added to the zinc by introducing calcium turnings into the molten zinc and maintaining them below the surface until they were dissolved in the zinc. After the calcium was alloyed with the zinc, the mixture was further heated and the alloy was then cast into a form suitable for use as anodes.

The zinc used was of commercial grade frequently employed for the manufacture of zinc anodes. The zinc contained about 0.07% of cadmium and 0.005% of iron. It also contained 0.10% of lead and the cast anodes, accordingly, contained these impurities in additionto the added calcium. For testing the anodes, 2. eye.- nide-zinc plating bath of the type recently developed for the deposition of brilliant zinc was made up as follows:

I Grams per liter Zinc cyanide (Zn(CN) 2) 60 Sodium cyanide (NaCN) 52.5 Sodium hydroxide (NaOH) 78 Molybdic acid (M003) 7.5

Using the above bath and the zinc-calcium Example II A zinc-calcium alloy anode was made up according to the procedure described in the above Example I, but using 0.6% of calcium. The zinc used was somewhat purer than that of the above example, containing only 0.05% of lead. In

.making the alloy 0.05% of lead was added to decrease the solubility of the anodes.

After the alloy was cast into a suitable form,

the anodes were tested in a bath such as that shown in Example I and were found to have an anode efiiciency of about 84%.

ficacious in preventing the deleterious action of lead and other such impurities that it is satisfactory to gain the benefits which may be had by using lead and saving on the more expensive calcium.

As indicated above. the calcium probably acts by forming an alkali-insoluble calcium hydrate or cyanide film which surrounds the particles of lead and drags them to the bottom of the bath where they can do no harm. It will be observed, 01' course, that there must be sufilcient calcium present since otherwise the lead would be free to exercise a deleterious influence.

Example III A zinc-calcium alloy anode was made by using electrolytic zinc and 1.07% of calcium. The alloy was made following the procedure used in the above Example I, the electrolytic zinc being melted directly in the graphite crucible. The zinccalcium alloy anodes so produced were tested in a bright zinc bath such as that shown above in Example I and it was found that the anode efficiency was about 82%.

While I have shown specific conditions in the foregoing for the production of zinc-calcium alloy anodes, it will be understood that these may be made in various ways according to known metallurgical practices. It is to be observed, however, that after the calcium is added to the zinc, the mixture should be heated to a temperature not substantially lower than about 800 C. In making up one set of electrolytic zinc anodes this precaution was not observed, the metal being poured at a temperature somewhere slightly above 600 C. and the allow anodes so produced had an elfficiency only slightly lower than that of electrolytic zinc itself. The specific temperature required to eflect the required change in alloy properties can readily be determined in each specific instance by a few simple trials. The electrolytic zinc alloy anodes mentioned were remelted and heated to a temperature of 800 C. and it was then found that the calcium exerclsede its usual profound effect upon the anode efllciency.

. From the foregoing it will be'evident that widely varying amounts of calcium may be used depending upon the eflect desired and upon the character of the zinc which it is desired to modify. By the use of suitable amounts of calcium it is I entirely possible to adjust the anode eillciency to correspond to the cathode efllciency of substantially any commercial cyanide-zinc electroplating bath nad it is principally in this connection that "the invention will be round valuable. It will be I understood. however, that the beneflcial'efi'ects or calcium may be obtained by the use of small amounts of calcium which will prevent the deleterious influence of metal impurities in the anode even without obtaining a very great reduction in the anode eillciency. Very small amounts of calcium may therefore be used to advantage.

Larger amounts of calcium may of course be used, tho it will be understood that it will not be commercial to use large amounts. In general, of course, the upper limit on the amount of calcium used is determined by the cathode efilciency of the baths in which the anodes are to be employed and by economic considerations.

While, as above indicated, widely varying amounts of calcium may be used, it will generally be found desirable to use from about 0.05% to about 5.0% of calcium. More specifically, it will.

usually be found that the desired anode efficiency and character can be obtained using from about 0.25% to 1.5% of calcium with electrolytic zinc or about 0.25% to 1.0% of calcium with commercial zinc such as that shown in Examples I and II and obtained by distillation processes.

The zinc-calcium alloy anodes of my invention have been particularly discussed above in conjunction with their use in a particular. cyanidezinc plating bath, but it will be understood that the type of anodes of my invention may advantaneously be employed in conjunction with various other such cyanide-zinc electroplating baths.

I claim:

1. In a process for the electrodeposition of zinc from a cyanide-zinc plating bath, the step comprising employing a zinc anode which has an efficiency substantially corresponding to the cathode emciency of the bath, the anode efilciency be- 'ing decreased by the inclusion in the anode of a small amount of calcium.

2. In a process for the electrodeposition of zinc from a cyanide-zincplating bath, the step comprising employing a zinc anode which contains from about 0.05 to 5.0 per cent of calcium the calcium effecting a diminution of the anode efficiency.

3. In a process for the electrodeposition of zinc from a cyanide-zinc plating bath, the step comprising employing a zinc anode which contains lead and a small amount of calcium, the calcium eflecting a diminution of the anode efilciency. I

4. In a process for the electrodeposition of zinc from a cyanide-zinc plating bath, the step comprising employing an electrolytic zinc anode which contains a small amount of calcium, the calcium eflecting a diminution of the anode emciency.

5. In a process for the electrodeposition of zinc from a cyanide-zinc plating bath, the step comprising employing a zinc anode which contains lead as an impurity and which contains from about 0.25 to 1.0 per cent of calcium, the calcium effecting a diminution of the anode eillciency.

6. In a process for the electrodeposition of zinc from a cyanide-zinc plating bath, the step comprising employing an electrolytic zinc anode which contains from about 0.25 to 1.5 per cent of calcium, the calcium efi'ectin'g a diminution of the anode emciency.

RICHARD 0. HULL. 

